This application claims the priority of German Application No. 199 20 093.9, filed May 3, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for controlling the evaporator temperature of a vehicle air conditioner and, more particularly, to a method for controlling the evaporator temperature of a vehicle air conditioner having an evaporator, a compressor, a condenser and an expansion element, in which the evaporator temperature is adjusted as a function of operating conditions.
Vehicle air conditioners are generally known. As a rule, a compressor compresses a gaseous refrigerant. When the refrigerant flows through a condenser, an expansion element and an evaporator, the air, which also flows through the evaporator, is cooled. The lowering of the air temperature usually takes place to a temperature of approximately 1xc2x0 C. to 3xc2x0 C. Since this temperature is usually below the dew point of the air taken in for cooling, water condenses out at the evaporator and the air is dried. Subsequently, in most operating conditions, the cooled air is heated again so that it reaches the interior in an optimally temperature-moderated manner. The cooling to 1xc2x0 C. to 3xc2x0 C., which takes place irrespective of the air-conditioning demand, therefore requires too much refrigerating capacity.
It will therefore be an object of the present invention to always operate the air conditioner such that the air is cooled only to a temperature required at the time. For this purpose, it is necessary to adjust the evaporator temperature, specifically according to the requirements in the vehicle.
From German Patent document DE 196 08 015 C, an air mixing system for a vehicle air conditioner is known for lowering the energy consumption, in case which, among other things, the specific fresh air enthalpy is taken into account. In particular, the smallest specific enthalpy difference is set at a heat exchanger. For finding the specific enthalpy, the temperature and the relative humidity of the air are measured.
However, for taking into account all operating conditions as well as the comfort and safety aspects in the case of a vehicle, it is necessary to further develop such an air conditioner.
It is an object of the present invention therefore to provide a method for operating an air conditioner which takes into account all conditions with respect to comfort and safety.
This object is achieved by a method for controlling the evaporator temperature of a vehicle air conditioner having an evaporator, a compressor, a condenser and an expansion element, in which the evaporator temperature is adjusted as a function of operating conditions. A first evaporator temperature value is determined with respect to a desired air humidity. A second evaporator temperature value is determined for ensuring a sufficient cooling capacity. The lowest of the two evaporator temperature values is selected as the evaporator temperature to be set.
For avoiding an unnecessary energy consumption, the evaporator temperature is adjusted to a temperature value which takes into account the comfort aspects as well as the safety aspects. On the one hand, an evaporator temperature value with respect to a certain air humidity is determined. On the other hand, an evaporator temperature value for ensuring a sufficient cooling capacity is determined. Then the lowest of the above-mentioned evaporator temperature values is selected for use.
The evaporator temperature value which is determined with respect to a desired air humidity preferably is to be determined as a function of the misting criteria of the vehicle windows and comfort criteria. As a result, it is ensured, on the one hand, that the driver""s view is not impaired. On the other hand, a pleasant condition, which is not too dry, of the air in the interior is ensured for the vehicle occupants. For both cases, a maximal acceptable air humidity is obtained in the vehicle. Thus, the dew point of the air must not fall below the temperature of the interior surfaces of the windows, which must be computed for this purpose. As the comfort limit value, a maximally permissible enthalpy of the air in the vehicle interior is used which is determined from the humidity and the temperature.
The humidity of the interior is, among other things, a function of the occupants"" water vapor emission as well as of the amount and humidity of the blow-in air. The evaporator temperature, among other things, can influence the condition of the blow-in air. This evaporator temperature must be adjusted such that the humidity of the interior does not exceed the permissible limits for meeting the misting and comfort requirements.
The evaporator temperature value, which aims at a sufficient cooling capacity, is influenced by the desired blow-in temperature and the undesirable heating in the air guiding components that are connected behind the evaporator. Preferably, when determining the assigned evaporator temperature to ensure sufficient cooling, the interior temperature, the ambient temperature and/or the sun radiation are taken into account as influential factors for the undesirable heating.
A particularly preferred embodiment is characterized in that a change-over to the above-mentioned method takes place only when certain operating conditions exist. Thus, at critical operating conditions, an evaporator temperature is ensured which is always sufficiently low.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.